Load control device configured to operate in two-wire and three-wire modes

ABSTRACT

A load control device coupled between an AC power source and an electrical load may operate in a three-wire mode or a two-wire mode based on whether the load control device is connected to a neutral side of the AC power source. The load control device may further comprise first and second zero-cross detect circuits to be respectively used in the two-wire mode or the three-wire mode, and a neutral wire detect circuit configured to generate a neutral-wire detect signal indicating whether the load control device is connected to the neutral side of the AC power source. A control circuit of the load control device may determine whether the load control device should operate in the two-wire mode or in the three-wire mode in response to the neutral-wire detect signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional U.S. PatentApplication No. 62/832,476, filed Apr. 11, 2019, Provisional U.S. PatentApplication No. 62/826,406, filed Mar. 29, 2019, and Provisional U.S.Patent Application No. 62/773,803, filed Nov. 30, 2018, the disclosuresof which are incorporated herein by reference in their entireties.

BACKGROUND

Prior art load control devices, such as dimmer switches, may be coupledin series electrical connection between an alternating-current (AC)power source and a lighting load for controlling the amount of powerdelivered from the AC power source to the lighting load. A standarddimmer switch may typically comprise a bidirectional semiconductorswitch, e.g., a thyristor (e.g., such as a triac) or two field-effecttransistors (FETs) in anti-series connection. The bidirectionalsemiconductor switch may be coupled in series between the AC powersource and the load and is controlled to be conductive andnon-conductive for portions of a half cycle of the AC power source tothus control the amount of power delivered to the electrical load.Generally, dimmer switches may use either a forward phase-controldimming technique or a reverse phase-control dimming technique in orderto control when the bidirectional semiconductor switch is renderedconductive and non-conductive to thus control the power delivered to theload. The dimmer switch may comprise a toggle actuator for turning thelighting load on and off and an intensity adjustment actuator foradjusting the intensity of the lighting load. Examples of prior artdimmer switches are described in greater detail is commonly-assignedU.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROLDEVICE; and U.S. Pat. No. 6,969,959, issued Nov. 29, 2005, entitledELECTRONIC CONTROL SYSTEMS AND METHODS; the entire disclosures of whichare incorporated by reference herein.

In order to save energy, high-efficiency lighting loads, such as, forexample, light-emitting diode (LED) light sources, are being used inplace of or as replacements for conventional incandescent or halogenlamps. High-efficiency light sources typically consume less power andprovide longer operational lives as compared to incandescent and halogenlamps. In order to illuminate properly, a load regulation circuit (e.g.,such as an electronic dimming ballast or an LED driver) may be coupledbetween the AC power source and the respective high-efficiency lightsource (e.g., the compact fluorescent lamp or the LED light source) forregulating the power supplied to the high-efficiency light source. Somehigh-efficiency lighting loads may be integrally housed with the loadregulation circuit in a single enclosure. Such an enclosure may have ascrew-in base that allows for mechanical attachment to standard Edisonsockets and provide electrical connections to the neutral side of the ACpower source and either the hot side of the AC power source or thedimmed-hot terminal of the dimmer switch (e.g., for receipt of thephase-control voltage).

A dimmer switch for controlling a high-efficiency light source may becoupled in series between the AC power source and the load regulationcircuit for the high-efficiency light source. Such a dimmer switch mayoperate in a two-wire mode or a three-wire mode, depending on whetherthe dimmer switch includes a neutral terminal and/or whether the neutralterminal is connected to a neutral side of the AC source. The loadregulation circuit may control the intensity of the high-efficiencylight source to the desired intensity in response to the conduction timeof the bidirectional semiconductor switch of the dimmer switch.

SUMMARY

As described herein, a load control device for controlling powerdelivered from an AC power source to an electrical load may comprise ahot terminal, a dimmer-hot terminal, and a neutral terminal. The hotterminal may be adapted to be electrically coupled to a hot side of theAC power source. The dimmed-hot terminal may be adapted to beelectrically coupled to the electrical load while the neutral terminalmay be optionally connected to a neutral side of the AC power source.The load control device may further comprise a first zero-cross detectcircuit, a second zero-cross detect circuit, and a neutral wire detectcircuit. The first and second zero-cross detect circuits may beconfigured to detect a zero-crossing point of an AC mains line voltagegenerated by the AC power source, and the neutral wire detect circuitmay be configured to generate, based on a current conducted through thesecond zero-cross detect circuit, a neutral-wire detect signalindicating whether the neutral terminal is connected to the neutral sideof the AC power source.

A control circuit of the load control device may determine whether theload control device should operate in a two-wire mode or a three-wiremode based on the neutral wire detect signal, wherein the two-wire modemay correspond to the neutral terminal not being connected to theneutral side of the AC power source and the three-wire mode maycorrespond to the neutral terminal being connected to the neutral sideof the AC power source. The control circuit may determine thezero-crossing points of the AC mains line voltage in response to thefirst zero-cross detect circuit in the two-wire mode and in response tothe second zero-cross detect circuit in the three-wire mode.

The second zero-cross detect circuit described above may comprise anactive filter configured to remove one or more frequency components ofthe AC mains line voltage that are above a frequency threshold. Theactive filter may be configured as a full-wave filter circuit or ahalf-wave filter circuit. When configured as a half-wave filter circuit,the active filter may be characterized by one or more of the following.The active filter may be powered by a same power supply that also powersthe control circuit and/or other components of the load control device.The active filter may be referenced to circuit common. The active filtermay be configured to conduct a current through the electrical load onlyduring negative half-cycles of the AC mains line voltage.

Also described herein is a load control device coupled between an ACpower source and an electrical load. The load control device maycomprise a hot terminal, a dimmer-hot terminal and a neutral terminal.The load control device may further comprise a power supply capable ofconducting a charging current through the electrical load and aswitching circuit configured to be rendered conductive andnon-conductive to control when the charging current is conducted throughthe electrical load.

A control circuit of the load control device may determine whether theload control device should operate in a two-wire mode or a three-wiremode, wherein the two-wire mode may correspond to the neutral terminalnot being connected to the neutral side of the AC power source andwherein the three-wire mode may correspond to the neutral terminal beingconnected to the neutral side of the AC power source. Upon determiningthat the load control device should operate in the two-wire mode, thecontrol circuit may render the switching circuit conductive to allow thecharging current to be conducted through the electrical load duringpositive half-cycles of an AC mains line voltage generated by the ACpower source. Upon determining that the load control device shouldoperate in the three-wire mode, the control circuit may render theswitching circuit non-conductive to prevent the charging current frombeing conducted through the electrical load during the positivehalf-cycles of the AC mains line voltage.

In addition, a load control device configured to execute a plurality ofdifferent power supply protection techniques (e.g., when operating inthe two-wire mode and/or when using a reverse phase-control dimmingtechnique) is also described herein. The load control device may beconfigured to control power delivered from an AC power source to anelectrical load. The load control device may comprise a controllablyconductive device adapted to be coupled in series with the electricalload, and a control circuit configured to render the controllablyconductive device conductive and non-conductive to control a loadcurrent conducted through the electrical load. The control circuit maybe configured to adjust an amount of power delivered to the electricalload by adjusting a present phase angle of the controllably conductivedevice between a low-end phase angle and a high-end phase angle. Theload control device may comprise a power supply configured to receive arectified voltage and to generate a supply voltage for powering thecontrol circuit by conducting a charging current through the electricalload when the controllably conductive device is non-conductive. Thepower supply may comprise a bus capacitor configured to charge from therectified voltage through a diode to generate a bus voltage. The controlcircuit may be configured to decrease the high-end phase angle when themagnitude of the rectified voltage is less than a first threshold, anddecrease the present phase angle when the magnitude of the bus voltageis less than a second threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example load control device(e.g., a dimmer switch) for controlling the amount of power delivered toan electrical load, such as, a lighting load.

FIG. 2 is a simplified partial schematic diagram of another example loadcontrol device showing a neutral wire detect circuit.

FIGS. 3A and 3B show simplified waveforms that illustrate the operationof the load control device of FIG. 2.

FIG. 4 shows a simplified flowchart of an example startup procedure thatmay be executed by a control circuit of a load control device.

FIGS. 5A and 5B show a simplified flowchart of an example controlprocedure that may be executed by a control circuit of a load controldevice.

FIG. 6 is a simplified block diagram of an example load control device(e.g., a dimmer switch) for controlling the amount of power delivered toan electrical load, such as, a lighting load.

FIG. 7 is a state diagram illustrating the operation of a controlcircuit of a load control device during an example control procedure.

FIG. 8 is a flowchart of an example phase-control adjustment procedurethat may be executed by a control circuit of a load control device.

FIG. 9 is a flowchart of an example countdown timer procedure that maybe executed by a control circuit of a load control device.

FIG. 10 is a flowchart of an example high-end trim adjustment procedurethat may be executed by a control circuit of a load control device.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an example load control device100 (e.g., a dimmer switch) for controlling the amount of powerdelivered to an electrical load, such as, a lighting load 102. The loadcontrol device 100 may include a hot terminal H coupled to a hot side ofan alternating-current (AC) power source 104 for receiving an AC mainsline voltage VAC, and a dimmed-hot terminal DH coupled to the lightingload 102. The load control device 100 may also include a neutralterminal N that may be adapted to be coupled (e.g., optionally coupled)to a neutral side of the AC power source 104. For example, the loadcontrol device 100 may be configured to operate in a two-wire mode whenthe neutral terminal N is not connected to the neutral side of the ACpower source 104 and in a three-wire mode when the neutral terminal N isconnected to the neutral side of the AC power source.

The load control device 100 may comprise a controllably conductivedevice 110, such as two field-effect transistors (FETs) Q112, Q114 thatmay be coupled in anti-series connection between the hot terminal andthe dimmed-hot terminal DH. The junction of the FETs may be coupled tocircuit common. The load control device 100 may comprise a controlcircuit 115, e.g., a digital control circuit, for controlling the FETsQ112, Q114 to conduct a load current I_(LOAD) through the lighting load102. The control circuit 115 may include one or more of a processor(e.g., a microprocessor), a microcontroller, a programmable logic device(PLD), a field programmable gate array (FPGA), an application specificintegrated circuit (ASIC), or any suitable controller or processingdevice. The load control device 100 may comprise a memory (not shown)configured to store operational characteristics of the load controldevice. The memory may be implemented as an external integrated circuit(IC) or as an internal circuit of the control circuit 115.

The control circuit 115 may generate first and second drive signalsV_(DR1), V_(DR2) that may be coupled to the gates of the respective FETsQ112, Q114 via first and second gate drive circuits 116, 118,respectively. When the controllably conductive device 110 is renderedconductive during the positive half-cycles of the AC power source 104,the load current I_(LOAD) may be conducted through the drain-sourcechannel of the first FET Q112 and the body diode of the second FET Q114.When the controllably conductive device 110 is rendered conductiveduring the negative half-cycles of the AC power source 104, the loadcurrent I_(LOAD) may be conducted through the drain-source channel ofthe second FET Q114 and the body diode of the first FET Q112.

The load control device 100 may comprise a user interface 117, which maycomprise, for example, one or more actuators (e.g., buttons) forreceiving user inputs and/or one or more visual indicators for providinguser feedback. For example, the user interface 117 may comprise a toggleactuator and an intensity adjustment actuator (e.g., such as a slidercontrol or a pair of raise and lower buttons) for controlling thelighting load 102. The control circuit 115 may be configured to controlthe controllably conductive device 110 to control the amount of powerdelivered to the lighting load 102 in response to actuations of theactuators of the user interface 117. For example, the control circuit115 may be configured to turn the lighting load 102 on and off inresponse to actuations of the toggle actuator. The control circuit 115may also be configured to control the amount of power delivered to thelighting load 102 to adjust a desired intensity L_(DES) of the lightingload between a high-end intensity L_(HE) (e.g., 90-100%) and a low-endintensity L_(LE) (e.g., 0.1-10%) in response to actuations of theintensity adjustment actuator. In addition, the user interface 117 mayalso comprise one or more light-emitting diodes (LEDs) for illuminatingthe visual indicators, for example, to provide a visual indication of astatus and/or a present intensity of the lighting load 102, and/or avisual indication of a selected preset. For example, the user interface117 may comprise a vertically-oriented linear array of visualindicators. The control circuit 115 may be coupled to the LEDs forilluminating the visual indicators of the user interface 117 to providefeedback.

The load control device 100 may comprise a communication circuit 119.The communication circuit 119 may comprise a wireless communicationcircuit, such as, for example, a radio-frequency (RF) transceivercoupled to an antenna for transmitting and/or receiving RF signals, anRF transmitter for transmitting RF signals, an RF receiver for receivingRF signals, or an infrared (IR) transmitter and/or receiver fortransmitting and/or receiving IR signals. The communication circuit 119may comprise a wired communication circuit configured to be coupled to awired control link, for example, a digital communication link and/or ananalog control link, such as a 0-10V control link or a pulse-widthmodulated (PWM) control link. In addition, the communication circuit 118may be coupled to the electrical wiring connected to the load controldevice 100 for transmitting a control signal via the electrical wiringusing, for example, a power-line carrier (PLC) communication technique.The control circuit 115 may be configured to turn the lighting load 102on and off, and adjust the desired intensity L_(DES) of the lightingload in response to messages (e.g., digital messages) received via thecommunication circuit 119.

The load control device 100 may include a power supply 120. The powersupply 120 may generate first direct-current (DC) supply voltageV_(CC1), e.g., for powering the control circuit 115 and the otherlow-voltage circuitry of the load control device 100, a second DC supplyvoltage V_(CC2), e.g., for powering the drive circuits 116, 118 to drivethe FETs Q112, Q114, and/or a third DC supply voltage V_(CC3) (e.g., anisolated DC supply voltage). For example, the power supply 120 maycomprise an isolated power supply, and may comprise a transformer forgenerating the third isolated DC supply voltage V_(CC3). The powersupply 100 may be configured to conduct a charging current through thedimmed-hot terminal DH and/or the neutral terminal N depending onwhether the neutral terminal N is connected to the neutral side of theAC power source 104 or not. The load control device 100 may comprise afirst diode D121 coupled between the hot terminal H and an input of thepower supply 120, a second diode D122 coupled between the dimmed-hotterminal DH and the input of the power supply 120, and a third diodeD123 coupled between the neutral terminal N and the input of the powersupply 120. When the neutral terminal N is not connected to the neutralside of the power supply 120, the power supply 120 may be coupled to theAC power source 104 through a full-wave rectifier bridge that includesthe first and second diodes D121, D122, and the body diodes of the FETsQ112, Q114. When the neutral terminal N is connected to the neutral sideof the power supply 120, the power supply 120 may be coupled to the ACpower source 104 through a full-wave rectifier bridge that includes thefirst and third diodes D121, D123, and the body diodes of the FETs Q112,Q114. The full-wave rectifier bridges (e.g., including the first diodeD121, the second diode D122, the third diode D123, and/or the bodydiodes of the FETs Q112, Q114) may be configured to receive a voltagedeveloped across the controllably conductive device 110 and to generatea rectified voltage V_(RECT) at the input of the power supply.

The control circuit 115 may be configured to determine times ofzero-crossing points of the AC mains line voltage V_(AC) of the AC powersource 104. The control circuit 115 may then render the FETs Q112, Q114conductive and/or non-conductive at predetermined times (e.g., at afiring time or firing angle) relative to the zero-crossing points of theAC mains line voltage V_(AC) to generate a phase-control voltage V_(PC)using a phase-control dimming technique (e.g., a forward phase-controldimming technique and/or a reverse phase-control dimming technique). Forexample, the control circuit 115 may use the forward phase-controldimming technique to control inductive loads, and may use the reversephase-control dimming technique to control capacitive loads. Examples ofdimmers are described in greater detail in commonly-assigned U.S. Pat.No. 7,242,150, issued Jul. 10, 2007, entitled DIMMER HAVING A POWERSUPPLY MONITORING CIRCUIT; U.S. Pat. No. 7,546,473, issued Jun. 9, 2009,entitled DIMMER HAVING A MICROPROCESSOR-CONTROLLED POWER SUPPLY; andU.S. Pat. No. 8,664,881, issued Mar. 4, 2014, entitled TWO-WIRE DIMMERSWITCH FOR LOW-POWER LOADS, the entire disclosures of which areincorporated by reference herein.

The control circuit 115 may be configured to adjust a phase angle (e.g.,a conduction time) of the controllably conductive device 110 eachhalf-cycle to control the amount of power delivered to the lighting load102 and the intensity of the lighting load. For example, the controlcircuit 115 may be configured to adjust a present phase angle θ_(PRES)of the controllably conductive device 110 to adjust the intensity of thelighting load 102 to the desired intensity L_(DES) (e.g., as set by theintensity adjustment actuator of the user interface 117). Using theforward phase-control dimming technique, the control circuit 115 mayrender the controllably conductive device 110 non-conductive at thebeginning of each half cycle, and render the controllably conductivedevice conductive at a firing time (e.g., as determined from the presentphase angle θ_(PRES)) during the half cycle. Using the reversephase-control dimming technique, the control circuit 115 may render thecontrollably conductive device 110 conductive at the beginning of eachhalf cycle, and render the controllably conductive device non-conductiveat a firing time (e.g., as determined from the present phase angleθ_(PRES)) during the half cycle, after which the control circuit maymaintain the controllably conductive device non-conductive for the restof the half cycle.

The load control device 100 may be programmed by a user duringinstallation to use the forward phase-control dimming technique or thereverse phase-control dimming technique during operation. For example,the user may set the phase-control dimming technique using an advancedprogramming mode. The control circuit 115 may be configured to enter theadvanced programming mode in response to one or more actuations of theactuators of the user interface 117. A load control device having anadvanced programming mode is described in greater detail incommonly-assigned U.S. Pat. No. 7,190,125, issued Mar. 13, 2007,entitled PROGRAMMABLE WALLBOX DIMMER, the entire disclosure of which ishereby incorporated by reference.

The control circuit 115 may employ a load detection process fordetermining a load type of lighting load 102 and use the phase-controldimming technique that is best suited for that load type. For example,the control circuit 115 may detect that the lighting load 102 isinductive, and may determine to use the forward phase-control dimmingtechnique. For example, upon initial power up, the control circuit 115may begin using the reverse phase-control dimming technique and maymonitor the voltage across the lighting load 102 using a voltage monitorcircuit (not shown) during the load detection process. In the event thatthe control circuit 115 detects an overvoltage condition (e.g., avoltage spike or ring-up condition) across the lighting load 102, theload control device may determine that the lighting load has inductivecharacteristics, and may begin using the forward phase-control dimmingtechnique. Otherwise, the control circuit 115 may continue to use thereverse-phase control dimming technique. Similarly, upon initial powerup, the control circuit 115 may begin using the forward phase-controldimming technique and may subsequently decide to switch to thereverse-phase control dimming technique (e.g., upon detecting that thelighting load has capacitive characteristics) or to continue to use theforward phase-control dimming technique. An example of a load controldevice that uses a load detection process is described in greater detailin commonly-assigned U.S. Pat. No. 9,489,005, issued Nov. 8, 2016,entitled METHOD AND APPARATUS FOR PHASE-CONTROLLING A LOAD, the entiredisclosure of which is hereby incorporated by reference.

The load control device 100 may comprise a two-wire zero-cross detectcircuit 130 coupled across the first FET Q112 (e.g., between the hotterminal H and the dimmed hot terminal DH) for generating a two-wirezero-cross signal V_(2WZC). The load control device 100 may alsocomprise a three-wire zero-cross detect circuit 140 (e.g., coupledbetween the hot terminal H and the neutral terminal N) for generating athree-wire zero-cross signal V_(3WZC). The control circuit 115 may beconfigured to receive the two-wire zero-cross signal V_(2WZC) and/or thethree-wire zero-cross signal V_(3WZC), and to determine the times of thezero-crossing points of the AC mains line voltage V_(AC) in response tothe two-wire zero-cross signal V_(2WZC) and/or the three-wire zero-crosssignal V_(3WZC).

The load control device 100 may comprise a neutral wire detect circuit150 coupled in series with the neutral terminal N (e.g., between thethree-wire zero-cross detect circuit 140 and the neutral terminal N).The neutral wire detect circuit 140 may be configured to generate aneutral wire detect signal V_(NWD) in response to current flowingthrough the three-wire zero-cross detect circuit 140. The controlcircuit 115 may be configured to detect if the neutral terminal N isconnected to the neutral side of the AC power source 104 in response tothe neutral wire detect circuit 150. The control circuit 115 may beconfigured to determine whether to operate in the two-wire mode or thethree-wire mode in response to the neutral wire detect signal V_(NWD).For example, the control circuit 115 may be configured to automaticallydetermine to operate in the two-wire mode in response to detecting thatthe neutral terminal N is not connected to the neutral side of the ACpower source 104 and to operate in the three-wire mode in response todetecting that the neutral terminal N is connected to the neutral sideof the AC power source. For example, the control circuit 115 may beconfigured to automatically determine to operate in the two-wire mode orthe three-wire mode in response to the neutral wire detect signalV_(NWD) during a start-up procedure of the control circuit (e.g., whenpower is first applied to the load control device 100). In addition, thecontrol circuit 115 may monitor the neutral wire detect signal V_(NWD)during normal operation and determine to change between the two-wiremode and three-wire mode in response to the neutral wire detect signalV_(NWD).

The control circuit 115 may be configured to provide a visual indicationwhen the control circuit decides (e.g., automatically decides) tooperate in the two-wire or three-wire mode in response to the neutralwire detect signal V_(NWD) (e.g., to indicate when the neutral terminalN is connected to the neutral side of the AC power source 104). Thecontrol circuit 115 may blink one or more of the visual indicators ofthe user interface 122 when the control circuit decides to operate inthe two-wire or the three-wire mode. For example, the control circuit115 may control the user interface 122 to blink twice a top visualindicator of a vertically-oriented linear array of visual indicatorswhen the control circuit decides to operate in the three-wire mode. Thecontrol circuit 115 may be configured to not provide a visual indicationwhen the control circuit decides to operate in the two-wire mode. Sincethe control circuit 115 automatically decides to operate in the two-wiremode or the three-wire mode, the visual indication that the load controldevice 100 is operating in the two-wire mode or the three-wire mode maybe useful in determining how the load control device is operating.

The control circuit 115 may also be configured to provide a visualindication of the mode (e.g., two-wire mode or three-wire mode) that thecontrol circuit is operating in during the advanced programming mode(e.g., to indicate when the neutral terminal N is connected to theneutral side of the AC power source 104). The control circuit 115 may beconfigured to provide the visual indication of the mode when, forexample, the control circuit is first entering the advanced programmingmode. For example, the control circuit 115 may be configured to blinkone of the visual indicators a first number of times to indicate thetwo-wire mode and second number of times to indicate the three-wiremode. In addition, the control circuit 115 may be configured to providea visual indication of the phase-control dimming technique (e.g., theforward phase-control dimming technique or the reverse phase-controldimming technique) that is presently being used during the advancedprogramming mode. For example, the control circuit 115 may be configuredto blink one of the visual indicators (e.g., a different visualindicator than used to indicate the mode) a first number of times toindicate the forward phase-control dimming technique and second numberof times to indicate the reverse phase-control dimming technique.

The control circuit 115 may be configured to control the FETs Q112, Q114using both the forward phase-control dimming technique and/or thereverse phase-control dimming technique. When using the forwardphase-control dimming technique, the control circuit 115 may render oneor both of the FETs Q112, Q114 non-conductive (e.g., to cause thecontrollably conductive device 110 to be non-conductive) at thebeginning of each half-cycle of the AC mains line voltage, and thenrender one or both of the FETs Q112, Q114 conductive (e.g., to cause thecontrollably conductive device 110 to be conductive) at the firing timeduring the half-cycle after which the controllably conductive device 110may remain conductive until the end of the half-cycle. When using thereverse phase-control dimming technique, the control circuit may renderone or both of the FETs Q112, Q114 conductive (e.g., to cause thecontrollably conductive device 110 to be conductive) at the beginning ofeach half-cycle of the AC mains line voltage, and then render one orboth of the FETs Q112, Q114 non-conductive (e.g., to cause thecontrollably conductive device 110 to be non-conductive) at the firingtime during the half-cycle after which the controllably conductivedevice 110 may remain non-conductive until the end of the half-cycle.

The load control device 100 may comprise an impedance circuit 160, suchas a resistive load circuit (e.g., a “dummy” load circuit), fordischarging a capacitance of the lighting load 102, for example, afterthe control circuit 115 renders the FETs Q112, Q114 non-conductive atthe firing time when using the reverse phase-control dimming technique.The impedance circuit 160 may be coupled between the dimmed-hot terminalDH and the neutral terminal N (e.g., in parallel with the lighting load102). The impedance circuit may conduct a discharge current (e.g.,through the dimmed-hot terminal DH, the neutral wire detect circuit 150,and the neutral terminal N) in order to discharge the capacitance of thelighting load 102 after the FETs are rendered non-conductive. Forexample, the impedance circuit 160 may be characterized by a resistanceof approximately 68 kΩ.

The control circuit 115 may configured to determine the firing times forrendering the FETs Q112, Q114 conductive each half-cycle based on thetimes of zero-crossing points of the AC mains line voltage V_(AC) asdetermined from the two-wire zero-cross detect circuit 130 and/or thethree-wire zero-cross detect circuit 140. The two-wire zero-cross detectcircuit 130 may comprise a simple zero-cross detect circuit and maydrive the magnitude of the two-wire zero-cross signal V_(2WZC) lowtowards circuit common when the magnitude of the voltage across thefirst FET Q112 exceeds a predetermined threshold.

The three-wire zero-cross detect circuit 140 may comprise a moreadvanced zero-cross detect circuit that includes a filter circuit 142(e.g., a full-wave filter circuit) and/or a signal generation circuit144. The filter circuit 142 may comprise a low-pass active filtercircuit (e.g., comprising one or more operational amplifiers), such as afourth-order Bessel filter. The filter circuit 142 may receive a signalthat represents the AC mains line voltage V_(AC), and may generate afiltered signal V_(F). The filter circuit 142 may operate tosubstantially remove from (or attenuate in) the filtered signal V_(F)frequency components of the AC mains line voltage V_(AC) that are abovethe fundamental frequency. The signal generation circuit 144 may receivethe filtered signal V_(F) and generate the three-wire zero-cross signalV_(3WZC). Examples of a zero-cross detect circuit having a filtercircuit are described in greater detail in U.S. Pat. No. 6,091,205,issued Jul. 18, 2000, entitled PHASE CONTROLLED DIMMING SYSTEM WITHACTIVE FILTER FOR PREVENTING FLICKERING AND UNDESIRED INTENSITY CHANGES,the entire disclosure of which is hereby incorporated by reference.

The filter circuit 142 and/or the signal generation circuit 144 mayreceive power from the DC supply voltage V_(CC3) (e.g., which may be anisolated DC supply voltage), and may be referenced to a differentreference point than the circuit common of the load control device 100(e.g., the junction of the FETs Q112, Q114). The filter circuit 142 maybe coupled between the hot terminal H and the neutral terminal N. Thefilter circuit 142 may be substantially the same as the circuit shown inFIG. 8A of previously-referenced U.S. Pat. No. 6,091,205. The filtercircuit 142 may also comprise an input circuit configured to scale andoffset the AC mains line voltage V_(AC) before being received by theoperational amplifiers of the filter circuit. Since the filter circuit142 receives a sinusoidal signal that is a scaled and offset version ofthe AC mains line voltage V_(AC), the three-wire zero-cross detectcircuit 140 may operate as a full-wave zero-cross detect circuit. Withthe filter circuit 142 configured in this manner, the filtered signalV_(F) may be a sinusoidal signal (e.g., a full-wave sinusoidal signal)at the fundamental frequency of the AC mains lines voltage V_(AC) (e.g.,without high-frequency components). In examples (e.g., when the signalgeneration circuit 144 is not referenced to circuit common), the signalgeneration circuit 144 may comprise an optocoupler circuit at its outputfor coupling the three-wire zero-cross signal V_(3WZC) to the controlcircuit 115.

Due to a delay introduced by the filter circuit 142, the filtered signalV_(F) may be characterized by a phase delay with respect to the AC mainsline voltage V_(AC). Different filter circuits may produce differentphase delays. For example, a full-wave filter circuit (e.g., the filtercircuit 142 in FIG. 1) may produce a different phase delay than ahalf-wave filter circuit (e.g., the filter circuit 642 in FIG. 6). Thesignal generation circuit 144 may generate edges in the three-wirezero-cross signal V_(3WZC) (e.g., drive the three-wire zero-cross signalV_(3WZC) low towards circuit common) when the magnitude of the filteredvoltage V_(F) exceeds a predetermined threshold (e.g., the signalgeneration circuit 144 may be a simple zero-cross detect circuit).Because of the phase delay between the filtered signal V_(F) and the ACmains line voltage V_(AC), the edges of the three-wire zero-cross signalV_(3WZC) that indicate the zero-crossing points of the AC mains linevoltage V_(AC) may be offset (e.g., delayed) from the actualzero-crossing points of the AC mains line voltage V_(AC). The phasedelay may be pre-determined. The control circuit 115 may be configuredto store a value representing the phase delay in the memory 128 andprocess the three-wire zero-cross signal V_(3WZC) by factoring in thephase delay to determine the actual times of the zero-crossing points ofthe AC mains line voltage V_(AC).

When the load control device 100 is operating in the three-wire mode,the power supply 120 may be configured to conduct a charging currentthrough the neutral terminal N, the diode D123, the body diode of thefirst FET Q112, and the hot terminal H during the negative half-cyclesof the AC mains lines voltage V_(AC). During the positive half-cycles,the power supply 120 may be configured to conduct the charging currentthrough the hot terminal H, the diode D121, the body diode of the secondFET Q114, the dimmed-hot terminal DH, and the lighting load 102. In somecases, it may be desirable to prevent the charging current of the powersupply 120 from being conducted through the lighting load 102 during thethree-wire mode. The load control device 100 may comprise a controllableswitching circuit 129 (e.g., that may include a FET) coupled in serieswith the diode D121. The control circuit 115 may be configured togenerate a switch signal V_(SW) for rendering the controllable switchingcircuit 129 conductive and non-conductive. When the load control deviceis operating in the two-wire mode, the control circuit 115 may beconfigured to render the controllable switching circuit 129 conductive,such that the power supply 120 may conduct the charging current throughthe diode D121 during the positive half-cycles. When the load controldevice 100 is operating in the three-wire mode, the control circuit 115may be configured to render the controllable switching circuit 129non-conductive, such that the power supply 120 is not able to conductthe charging current through the diode D121 during the positivehalf-cycles. In the three-wire mode, the power supply 120 may only beable to conduct the charging current through the neutral terminal N inthe negative half cycles (e.g., the power supply operates a half-wavepower supply).

As described herein, the control circuit 115 may be configured todetermine whether to operate in the two-wire mode or the three-wire modein response to the neutral wire detect signal V_(NWD) generated by theneutral wire detection circuit 150. As such, the control circuit 115 maycontrol the switch signal V_(SW) for rendering the controllableswitching circuit 129 conductive and non-conductive based on the neutralwire detect signal V_(NWD). Alternatively or additionally, the controlcircuit 115 may be configured to determine whether it is operating inthe two-wire mode or the three-wire mode and to respectively render thecontrollable switching circuit 129 conductive and non-conductive basedon a user input (e.g., which may be received from a user input devicesuch as a button or a switch), based on a digital message received froman external device (e.g., from a system controller), and/or based onanother suitable mechanism.

FIG. 2 is a simplified partial schematic diagram of another example loadcontrol device 200 (e.g., the load control device 100 shown in FIG. 1 orthe load control device 600 shown in FIG. 6) for controlling the amountof power delivered to an electrical load, such as a lighting load (e.g.,the lighting load 102). FIGS. 3A and 3B show simplified waveforms thatillustrate the operation of the load control device 200. The loadcontrol device 200 may comprise a control circuit 215 (e.g., a digitalcontrol circuit) configured to control a controllably conductive device(not shown), such as, for example, the FETs Q112, Q114 of the loadcontrol device 100 shown in FIG. 1. The control circuit 215 may includeone or more of a processor (e.g., a microprocessor), a microcontroller,a programmable logic device (PLD), a field programmable gate array(FPGA), an application specific integrated circuit (ASIC), or anysuitable controller or processing device. The control circuit 215 may beconfigured to control the controllably conductive device using a forwardphase-control dimming technique or a reverse phase-control technique togenerate a phase-control voltage V_(PC) across the lighting load. Forexample, as shown in FIGS. 3A and 3B, the control circuit may controlthe controllably conductive device using the forward phase-controldimming technique for controlling the magnitude of the phase-controlvoltage to be approximately equal to zero volts for a non-conductiontime period T_(NC) at the beginning of each half-cycle and approximatelyequal to the magnitude of the AC line voltage for a conduction timeperiod T_(CON) at the end of each half-cycle.

The load control device 200 may comprise a two-wire zero-cross detectcircuit 230 that may be coupled across one or more of the FETs of thecontrollably conductive device. For example, the two-wire zero-crossdetect circuit 230 may be coupled across a first one of the FETs (e.g.,the first FET Q112 as shown in FIG. 1). The two-wire zero-cross detectcircuit 230 may be responsive to the voltage developed across the firstFET Q112 (e.g., when the first FET Q112 is non-conductive) and maygenerate a two-wire zero-cross signal V_(2WZC) that may indicatezero-crossing points of the AC mains line voltage V_(AC).

The load control device 200 may comprise a three-wire zero-cross detectcircuit 240 that may be coupled between a hot terminal H and a neutralterminal N of the load control device 200. The three-wire zero-crossdetect circuit 240 may be responsive to the AC mains line voltage V_(AC)and may generate a three-wire zero-cross signal V_(3WZC) that mayindicate zero-crossing points of the AC mains line voltage V_(AC). Thethree-wire zero-cross detect circuit 240 may comprise a filter circuit(not shown), such as a low-pass active filter circuit as described abovewith reference to FIG. 1.

The load control device 200 may also comprise a neutral wire detectcircuit 250 coupled in series with the three-wire zero-cross detectcircuit 240 between the hot terminal H and the neutral terminal N. Theneutral wire detect circuit 250 may generate a neutral wire detectsignal V_(NWD) in response to current flowing through the three-wirezero-cross detect circuit 240 (e.g., the neutral wire detect circuit 250may be a current-sensing device). The neutral wire detect signal V_(NWD)may indicate when the neutral terminal N is connected to the neutralside of the AC power source. The neutral wire detect circuit 250 maycomprise a diode D252 and a resistor R254 (e.g., having a resistance ofapproximately 4.74 kΩ) that may be coupled in parallel. The parallelcombination of the diode D252 and the resistor R254 may be coupledacross the base-emitter junction of a PNP bipolar junction transistorQ256. The transistor Q256 may be coupled to the base of an NPN bipolarjunction transistor Q266 via a diode D258 and a resistor R260 (e.g.,having a resistance of approximately 1 Me). The neutral wire detectcircuit 250 may also comprise a resistor R262 (e.g., having a resistanceof approximately 52.3 kΩ) and a capacitor C264 (e.g., having acapacitance of approximately 1000 pF) coupled in parallel across thebase-emitter junction of the transistor Q266. The collector of thetransistor Q266 may be coupled to the supply voltage V_(CC) through aresistor R268. The neutral wire detect signal V_(NWD) may be generatedat the junction of the transistor Q266 and the resistor R268, and may beprovided to the control circuit 215. In addition, the collector of thetransistor Q266 may be coupled to the supply voltage V_(CC) through aninternal pull-up resistor of the control circuit 215.

During the positive half-cycles of the AC mains line voltage V_(AC), thethree-wire zero-cross detect circuit 240 may conduct current from thehot terminal H through the parallel combination of the diode D252 andthe resistor R254 and out the neutral terminal N. At this time, thetransistor Q256 may be non-conductive. During the negative half-cycles,the three-wire zero-cross detect circuit 240 may conduct current fromthe neutral terminal N through the resistor R254 and out through the hotterminal H. The resistor R254 may generate a voltage that exceeds therated emitter-base voltage of the transistor Q256, thus causing thetransistor Q256 to become conductive. When conductive, the transistorQ256 may conduct current through the diode D258 and the resistors R260,R262. The resistor R262 may generate a voltage that exceeds the ratedbase-emitter voltage of the transistor Q266, thus causing the transistorQ266 to become conductive. When the transistor Q266 is conductive, thetransistor Q266 may drive the magnitude of the neutral wire detectsignal V_(NWD) down towards circuit common.

The control circuit 115 may be configured to determine whether tooperate in the two-wire mode or the three-wire mode in response to theneutral wire detect signal V_(NWD). FIG. 3A shows simplified waveformsthat illustrate the operation of the load control device 200 when theload control device is operating in the two-wire mode. FIG. 3B showssimplified waveforms that illustrate the operation of the load controldevice 200 when the load control device is operating in the three-wiremode.

When the neutral terminal N is not connected to the neutral side of theAC power source (e.g., when the load control device 200 is configured tooperate in the two-wire mode), the three-wire zero-cross detect circuit240 may not conduct current through the neutral wire detect circuit 250.Thus, the transistor Q266 of the neutral wire detect circuit 250 may benon-conductive and the magnitude of the neutral wire detect signalV_(NWD) may be pulled up towards the supply voltage V_(CC) in both thepositive and negative half-cycles of the AC mains line voltage V_(AC)(e.g., as shown in FIG. 3A). The control circuit 215 may be configuredto operate in the two-wire mode in response to detecting that themagnitude of the neutral wire detect signal V_(NWD) may be pulled uptowards the supply voltage V_(CC) (e.g., approximately maintained at thesupply voltage V_(CC)) in both the positive and negative half-cycles.

When operating in the two-wire mode, the control circuit 215 may controlthe FETs of the controllably conductive device in response to thetwo-wire zero-cross signal V_(2WZC). The two-wire zero-cross detectcircuit 230 may drive the magnitude of the two-wire zero-cross signalV_(2WZC) low towards circuit common when the magnitude of the voltageacross the first FET Q112 exceeds a predetermined threshold (e.g.,during the non-conduction time period T_(NC) as shown in FIG. 3A). Thecontrol circuit 215 may be configured to determine a zero-crossing pointat the beginning of each line cycle of the AC mains line voltage V_(AC)in response to detecting a falling edge of the two-wire zero-crosssignal V_(2WZC). When the neutral terminal N is not connected to theneutral side of the AC power source 104 (e.g., the neutral terminal N isfloating), the voltages at the inputs of the operational amplifiers ofthe filter circuit 142 may be at unknown magnitudes and the signalgenerator 144 may generate edges in the three-wire zero-cross signalV_(3WZC) at random times as shown in FIG. 3A (e.g., the edges of thethree-wire zero-cross signal V_(3WZC) may not always indicatezero-crossing points of the AC mains line voltage VAC when operating inthe two-wire mode).

When the neutral terminal N is connected to the neutral side of the ACpower source (e.g., when the load control device 200 is configured tooperate in the three-wire mode), the three-wire zero-cross detectcircuit 240 may conduct current through the neutral wire detect circuit250. For example, during the positive half-cycles of the AC mains linesvoltage V_(AC), the transistor Q256 of the neutral wire detect circuit250 may be non-conductive, and as a result, the transistor Q266 may benon-conductive causing the magnitude of the neutral wire detect signalV_(NWD) to be pulled up towards the supply voltage V_(CC). During thenegative half-cycles, the transistors Q256, Q266 may both be renderedconductive causing the magnitude of the neutral wire detect signalV_(NWD) to be pulled down towards circuit common (e.g., as shown in FIG.3B). The control circuit 215 may be configured to operate in thethree-wire mode in response to detecting changes in the magnitude of theneutral wire detect signal V_(NWD) during one or more line cycles of theAC mains line voltage. For example, the control circuit 215 may beconfigured to operate in the three-wire mode in response to detecting atransition of the magnitude of the neutral wire detect signal V_(NWD)between the supply voltage V_(CC) and circuit common (e.g., in responseto detecting that the magnitude of the neutral wire detect signalV_(NWD) is pulled down towards circuit common in the negativehalf-cycles or that the magnitude of the neutral wire detect signalV_(NWD) is pulled up towards the supply voltage V_(CC) in the positivehalf-cycles). The control circuit 215 may count the number of suchtransitions during a preconfigured number of line cycles and determinewhether the number of such transitions exceeds a threshold (e.g., apreconfigured threshold). The control circuit 215 may determine tooperate in the three-wire mode if the number of such transitions reachesor exceeds the threshold, and operate in the two-wire mode if the numberof such transitions is below the threshold.

When operating in the three-wire mode, the control circuit 215 maycontrol the FETs of the controllably conductive device in response tothe three-wire zero-cross signal V_(3WZC). The three-wire zero-crossdetect circuit 230 may generate edges in the three-wire zero-crosssignal V_(3WZC) that indicate the zero-crossing points of the AC mainsline voltage V_(AC). The frequency of the three-wire zero-cross signalV_(3WZC) may be approximately equal to the frequency of the AC mainsline voltage V_(AC). The control circuit 215 may be configured todetermine at least one zero-crossing point during each line cycle of theAC mains line voltage V_(AC) in response to detecting edges of thethree-wire zero-cross signal V_(3WZC). Because of the phase delaybetween the filtered signal V_(F) and the AC mains line voltage V_(AC),the edges of the three-wire zero-cross signal V_(3WZC) may be offset(e.g., delayed) from the actual zero-crossing points of the AC mainsline voltage V_(AC) by a phase delay period T_(PD) (e.g., as shown inFIG. 3B). The phase delay period T_(PD) may be predetermined, and thecontrol circuit 215 may be configured to store a value representing thephase delay period T_(PD) in memory. The control circuit 215 may beconfigured to determine the actual times of the zero-crossing points ofthe AC mains line voltage V_(AC), for example, by subtracting the phasedelay period from the times of the edges of the three-wire zero-crosssignal V_(3WZC).

FIG. 4 is a simplified flowchart of an example neutral wire detectprocedure 400 that may be executed by a control circuit of a loadcontrol device (e.g., the control circuit 115 of the load control device100, the control circuit 215 of the load control device 200, or thecontrol circuit 615 of the load control device 600) during startup.Using such a procedure, the control circuit may automatically determineto operate in the two-wire mode or the three-wire mode in response to aneutral wire detection signal (e.g., the neutral wire detect signalV_(NWD) described herein). For example, the control circuit may beconfigured to execute the procedure 400 during a start-up routine of thecontrol circuit (e.g., when power is first applied to the load controldevice 100) so that the control circuit may determine (e.g.,automatically determine) to operate in the two-wire mode or thethree-wire mode in response to the neutral wire detect signal V_(NWD).

Upon starting the procedure 400 at 410, the control circuit mayinitialize multiple variables at 412. For example, a variable n_(EDGE)may be defined to represent the number of edges (or transitions)detected in the magnitude of the neutral wire detect signal V_(NWD)during a number of line cycles of the AC mains line voltage V_(AC), andthe variable n_(LC) may be defined to represent the number of linecycles of the AC mains line voltage. At 412, the control circuit may setthe respective values of the variables n_(EDGE) and n_(LC) to zero. At414, the control circuit may determine whether an edge has been detectedin the neutral wire detect signal V_(NWD) during a current line cycle ofthe AC mains line voltage V_(AC). As described herein, such an edge maycorrespond to a transition of the magnitude of the neutral wire detectsignal V_(NWD) from a first value (e.g., a low value approximately equalto circuit common) to a second value (e.g., a high value approximatelyequal to the supply voltage V_(CC)), or vice versa. If the controlcircuit determines that an edge has been detected in the neutral wiredetect signal V_(NWD), the control circuit may increase the value of thevariable n_(EDGE) at 416, and may wait for the next line cycle at 418.The control circuit may also increment the value of the variable n_(LC)to keep track of the number of line cycles during which edge monitoringhas been performed.

At 424, the control circuit may compare the value of the variable n_(LC)to a preconfigured maximum value N_(LC-MAX) and determine whether thevalue of the variable n_(LC) has reached or exceeded the preconfiguredmaximum value N_(LC-MAX) (e.g., whether n_(LC) is equal to or greaterthan N_(LC-MAX)). The preconfigured maximum value N_(LC-MAX) mayrepresent a maximum number of line cycles during which the controlcircuit should test (e.g., monitor) to determine whether to operate inthe two-wire mode or the three-wire mode. The value of the preconfiguredmaximum value N_(LC-MAX) (e.g., five line cycles) may be predeterminedand stored in a memory of the load control device. If the controlcircuit determines that the value of the variable n_(LC) has reached orexceeded the preconfigured maximum value N_(LC-MAX), the control circuitmay further determine, at 426, whether the value of the variablen_(EDGE) has reached or exceeded another preconfigured maximum valueN_(EDGE-MAX) (e.g., whether n_(EDGE) is equal to or greater thanN_(EDGE-MAX)). The preconfigured maximum value N_(EDGE-MAX) mayrepresent a number of transitions or edges in the magnitude of theneutral wire detect signal V_(NWD) that, if detected within thepreconfigured maximum value N_(LC-MAX) line cycles, should cause thecontrol circuit to operate in the three-wire mode. The value of themaximum preconfigured value N_(EDGE-MAX) (e.g., three edges) may bepredetermined and stored in a memory of the load control device.

If the control circuit determines that the number of edges (e.g., threeor more edges) of the neutral wire detect signal V_(NWD) within themaximum number of line cycles (e.g., five line cycles) of the AC mainsline voltage V_(AC) has reached or exceeded the predetermined maximumvalue N_(EDGE-MAX) (e.g., three) at 426, the control circuit maydetermine to operate in the three-wire mode at 430, and may provide avisual indication that the control circuit is operating in thethree-wire mode at 432 (e.g., by blinking one or more visualindicators). If the control circuit detects less than the number ofedges (e.g., two or less edges) of the neutral wire detect signalV_(NWD) within the maximum number of line cycles (e.g., five linecycles) of the AC mains line voltage V_(AC), the control circuit maydetermine to operate in the two-wire mode at 428. After either 428 or432, the control circuit may exit the procedure 400.

If the control circuit detects no edge of the neutral wire detect signalV_(NWD) at 414, or if the control circuit determines that an end of thecurrent line cycle of the AC mains line voltage has not been reached at420, or if the preconfigured value of n_(LC-MAX) has not been reached at424, the control circuit may return to 414 to repeat the steps describedabove.

FIGS. 5A and 5B show a simplified flowchart of an example neutral wiredetect procedure 500 that may be executed by a control circuit of a loadcontrol (e.g., the control circuit 115 of the load control device 100,the control circuit 215 of the load control device 200, or the controlcircuit 615 of the load control device 600). Using such a procedure, thecontrol circuit may monitor the neutral wire detect signal V_(NWD)during normal operation (e.g., without resetting the load control deviceto cause a start-up routine to be executed) so that the control circuitmay determine to switch between the two-wire mode and three-wire mode inresponse to the neutral wire detect signal V_(NWD). For example, whenoperating in the two-wire mode, the control circuit may determine toswitch to the three-wire mode in response to detecting edges of theneutral wire detect signal V_(NWD) (e.g., three or more edges withinfive line cycles). When operating in the three-wire mode, the controlcircuit may determine to switch to the two-wire mode in response todetecting a lack of edges of the neutral wire detect signal V_(NWD)(e.g., no edges for at least five line cycles).

As shown in FIG. 5A, the control circuit may start the example neutralwire detect procedure 500 at 510. For example, the control circuit mayexecute the neutral wire detect procedure 500 periodically (e.g., every1 second) at 1010. Multiple variables may be defined. For example, avariable n_(EDGE) may be defined to represent the number of edgesdetected in the magnitude of the neutral wire detect signal V_(NWD)during a number of line cycles of the AC mains line voltage, and avariable n_(LC) may be defined to represent the number of such linecycles. At 512, the control circuit may determine whether the two-wiremode is being used. If the determination is that the two-wire mode isbeing used, the control circuit may further determine, at 514, whetherthe control circuit is operating in a neutral presence detect mode. Ifthe control circuit is not operating in the neutral presence detectmode, the control circuit may determine whether an edge (e.g., atransition between a first magnitude and a second magnitude) of theneutral wire detect signal V_(NWD) is detected at 516. If no edge isdetected, the control circuit may exit the neutral wire detect procedure500. If an edge is detected, the control circuit may enter the neutralpresence detect mode at 518 (e.g., the control circuit may set a flagindicating that it is in the neutral presence detect mode), and mayincrement the value of the variable n_(EDGE) (e.g., set the value ofn_(EDGE) to one) at 520 before exiting the neutral wire detect procedure500.

If the control circuit determines at 514 that the control circuit isoperating in the neutral presence detect mode (e.g., the control circuitmay have previously set a flag indicating that the control circuit is inthe neutral presence after detecting an edge of the neutral wire detectsignal V_(NWD) as described here), the control circuit may determine, at522, whether an edge (e.g., a transition between a first magnitude and asecond magnitude) of the neutral wire detect signal V_(NWD) is detected.If no edge is detected, the control circuit may set the value of thevariable n_(EDGE) to zero at 524 and exit the neutral presence detectmode at 526 before exiting the neutral wire detect procedure 500. If anedge of the neutral wire detect signal V_(NWD) is detected at 522, thecontrol circuit may increment the value of the variable n_(EDGE) at 528.At 530, the control circuit may compare the value of the variablen_(EDGE) to a preconfigured maximum value N_(EDGE-MAX1) and determinewhether the value of the variable n_(EDGE) has reached or exceeded thepreconfigured maximum value N_(EDGE-MAX1) (e.g., whether n_(EDGE) isequal to or greater than N_(EDGE-MAX1)). The value of the preconfiguredmaximum value N_(EDGE-MAX1) (e.g., three) may be predetermined andstored in a memory of the load control device.

If the control circuit determines at 530 that the variable n_(EDGE) isequal to or greater than the preconfigured maximum value N_(EDGE-MAX1),the control circuit may determine to operate in the three-wire mode at532, provide a visual indication that the control circuit is operatingin the three-wire mode at 534, and exit the neutral presence detect modeat 526, before the neutral wire detect procedure 500 exits.Alternatively, the control circuit may cause a reset of the load controldevice (e.g., cause a reset of the load control device at 532 instead ofdirectly switching to the three-wire mode at 532). Such a reset may leadto initialization and execution of a startup routine during which aneutral wire detect procedure (e.g., the neutral wire detect procedure400 shown in FIG. 4) may be executed by the control circuit to determinewhether the load control device should operate in the two-wire mode orthe three-wire mode.

If the control circuit determines at 512 that the control circuit is notoperating in the two-wire mode (e.g., the three-wire mode is used), thecontrol circuit may continue to 540 (shown in FIG. 5B) to determinewhether the control circuit is operating in a neutral absence detectmode. If the control circuit is not in the neutral absence detect mode,the control circuit may determine at 542 whether an edge (e.g., atransition between a first magnitude and a second magnitude) of theneutral wire detect signal V_(NWD) is detected. If an edge is detected,the control circuit may exit the neutral wire detect procedure 500. Ifthe control circuit detects no edge of the neutral wire detect signalV_(NWD), the control circuit may enter the neutral absence detect modeat 544, and may set the respective values of the variable n_(EDGE)(e.g., set the value of n_(EDGE) to zero) and the variable n_(LC) (e.g.,set the value of n_(EDGE) to one) at 546 before exiting the neutral wiredetect procedure 500.

If the control circuit determines at 540 that the control circuit isoperating in the neutral absence detect mode, the control circuit mayincrement the value of the variable n_(LC) (e.g., increment the value ofn_(LC) by one) at 548. At 550, the control circuit may determine whetheran edge (e.g., a transition between a first magnitude and a secondmagnitude) of the neutral wire detect signal V_(NWD) is detected. If noedge is detected, the control circuit may proceed to 554. If an edge ofthe neutral wire detect signal V_(NWD) is detected, the control circuitmay increment the value of the variable n_(EDGE) (e.g., increment thevalue of n_(EDGE) by one) at 552 before proceeding to 554. In eithercase, the control circuit may compare the value of the variable n_(LC)to a preconfigured maximum value N_(LC-MAX) at 554 to determine whetherthe value of the variable n_(LC) has reached or exceeded thepreconfigured maximum value N_(LC-MAX) (e.g., whether n_(LC) is equal toor greater than N_(LC-MAX)). The preconfigured maximum value N_(LC-MAX)may represent a number of line cycles for the control circuit to testdetermine whether the control circuit should operate in the two-wiremode (e.g., or cause a reset of the load control device to determinewhether to operate in the two-wire mode or the three-wire mode). Thevalue of the preconfigured maximum value N_(LC-MAX) (e.g., five linecycles) may be predetermined and stored in a memory of the load controldevice.

If the control circuit determines at 554 that the value of thepreconfigured maximum value N_(LC-MAX) has not been reached or exceeded,the control circuit may exit the neutral wire detect procedure 500. Ifthe control circuit determines at 554 that the value of thepreconfigured maximum value N_(LC-MAX) has been reached or exceeded, thecontrol circuit may further determine, at 556, whether the value of thevariable n_(EDGE) is equal to or greater than a preconfigured maximumvalue N_(EDGE-MAX2) (e.g., whether n_(EDGE) is equal to or greater thanN_(EDGE-MAX2)). The value of the preconfigured maximum valueN_(EDGE-MAX2) (e.g., three) may be predetermined and stored in a memoryof the load control device. The value of the preconfigured maximum valueN_(EDGE-MAX2) may be the same as or may be different from the value ofthe preconfigured maximum value N_(EDGE-MAX1).

If the control circuit determines at 556 that the variable n_(EDGE) isless than the preconfigured value of the preconfigured maximum valueN_(EDGE-MAX2), the control circuit may determine to operate in thetwo-wire mode at 558 and exit the neutral absence detect mode at 560,before the neutral wire detect procedure 500 exits. Alternatively, thecontrol circuit may cause a reset of the load control device (e.g.,cause a reset of the load control device at 558 instead of directlyswitching to the two-wire mode at 558). Such a reset may lead to theinitialization and execution of a startup routine during which a neutralwire detect procedure (e.g., the neutral wire detect procedure 400) maybe executed by the control circuit to determine whether the load controldevice should operate in the two-wire mode or the three-wire mode. Ifthe control circuit determines at 556 that the variable n_(EDGE) isequal to or greater than the preconfigured maximum value N_(EDGE-MAX2),the control circuit may exit the neutral absence detect mode at 560, andthe neutral wire detect procedure 500 may exit

FIG. 6 is a simplified block diagram of an example load control device600 (e.g., a dimmer switch) for controlling the amount of powerdelivered to an electrical load, such as, a lighting load 602. The loadcontrol device 600 may include a hot terminal H coupled to a hot side ofan alternating-current (AC) power source 604 for receiving an AC mainsline voltage V_(AC), and a dimmed-hot terminal DH coupled to thelighting load 602. The load control device 600 may also include aneutral terminal N that may be adapted to be coupled (e.g., optionallycoupled) to a neutral side of the AC power source 604. For example, theload control device 600 may be configured to operate in a two-wire modewhen the neutral terminal N is not connected to the neutral side of theAC power source 604 and in a three-wire mode when the neutral terminal Nis connected to the neutral side of the AC power source.

The load control device 600 may comprise a controllably conductivedevice 610 (e.g., such as the field-effect transistors (FETs) Q612,Q614) that may be coupled in anti-series connection between the hotterminal and the dimmed-hot terminal DH. The junction of the FETs may becoupled to circuit common. The load control device 600 may comprise acontrol circuit 615, e.g., a digital control circuit, for controllingthe controllably conductive device 610 to conduct a load currentI_(LOAD) through the lighting load 602. The control circuit 615 mayinclude one or more of a processor (e.g., a microprocessor), amicrocontroller, a programmable logic device (PLD), a field programmablegate array (FPGA), an application specific integrated circuit (ASIC), orany suitable controller or processing device. The load control device600 may comprise a memory (not shown) configured to store operationalcharacteristics of the load control device. The memory may beimplemented as an external integrated circuit (IC) or as an internalcircuit of the control circuit 615.

The control circuit 615 may generate first and second drive signalsV_(DR1), V_(DR2) that may be coupled to the gates of the respective FETsQ612, Q614 via first and second gate drive circuits 616, 618,respectively. When the controllably conductive device 610 is renderedconductive during the positive half-cycles of the AC power source 604,the load current I_(LOAD) may be conducted through the drain-sourcechannel of the first FET Q612 and the body diode of the second FET Q614.When the controllably conductive device 610 is rendered conductiveduring the negative half-cycles of the AC power source 604, the loadcurrent I_(LOAD) may be conducted through the drain-source channel ofthe second FET Q614 and the body diode of the first FET Q612.

The load control device 600 may comprise a user interface 617, which maycomprise, for example, one or more actuators (e.g., buttons) forreceiving user inputs and/or one or more visual indicators for providinguser feedback. For example, the user interface 617 may comprise a toggleactuator and an intensity adjustment actuator (e.g., such as a slidercontrol or a pair of raise and lower buttons) for controlling thelighting load 602. The control circuit 615 may be configured to controlthe controllably conductive device 610 to control the amount of powerdelivered to the lighting load 602 in response to actuations of theactuators of the user interface 617. For example, the control circuit615 may be configured to turn the lighting load 602 on and off inresponse to actuations of the toggle actuator. The control circuit 615may also be configured to control the amount of power delivered to thelighting load 602 to adjust a desired intensity L_(DES) of the lightingload between a high-end intensity L_(HE) (e.g., 90-100%) and a low-endintensity L_(LE) (e.g., 0.1-10%) in response to actuations of theintensity adjustment actuator. In addition, the user interface 617 mayalso comprise one or more light-emitting diodes (LEDs) for illuminatingthe visual indicators, for example, to provide a visual indication of astatus and/or a present intensity of a lighting load, and/or a visualindication of a selected preset. For example, the user interface 617 maycomprise a vertically-oriented linear array of visual indicators. Thecontrol circuit 615 may be coupled to the LEDs for illuminating thevisual indicators of the user interface 617 to provide feedback.

The load control device 600 may comprise a communication circuit 619.The communication circuit 619 may comprise a wireless communicationcircuit, such as, for example, a radio-frequency (RF) transceivercoupled to an antenna for transmitting and/or receiving RF signals, anRF transmitter for transmitting RF signals, an RF receiver for receivingRF signals, or an infrared (IR) transmitter and/or receiver fortransmitting and/or receiving IR signals. The communication circuit 619may comprise a wired communication circuit configured to be coupled to awired control link, for example, a digital communication link and/or ananalog control link, such as a 0-10V control link or a pulse-widthmodulated (PWM) control link. In addition, the communication circuit 118may be coupled to the electrical wiring connected to the load controldevice 600 for transmitting a control signal via the electrical wiringusing, for example, a power-line carrier (PLC) communication technique.The control circuit 615 may be configured to turn the lighting load 602on and off, and adjust the desired intensity L_(DES) of the lightingload in response to messages (e.g., digital messages) received via thecommunication circuit 619.

The load control device 600 may include a power supply 620. The powersupply 620 may generate a first direct-current (DC) supply voltageV_(CC1) (e.g., 3.3V), e.g., for powering the control circuit 615 and/orthe other low-voltage circuitry of the load control device 600. Thepower supply 620 may generate a second direct-current (DC) supplyvoltage V_(CC2) (e.g., 12V), e.g., for powering the drive circuits 616,618 to drive the FETs Q612, Q614. The load control device 600 maycomprise a first diode D621 coupled between the hot terminal H and aninput of the power supply 620, a second diode D622 coupled between thedimmed-hot terminal DH and the input of the power supply 620, and athird diode D623 coupled between the neutral terminal N and the input ofthe power supply 620. When the neutral terminal N is not connected tothe neutral side of the power supply 620, the power supply 620 may becoupled to the AC power source 604 through a full-wave rectifier bridgethat includes the first and second diodes D621, D622, and the bodydiodes of the FETs Q612, Q614. When the neutral terminal N is connectedto the neutral side of the power supply 620, the power supply 620 may becoupled to the AC power source 604 through a full-wave rectifier bridgethat includes the first and third diodes D621, D623, and the body diodesof the FETs Q612, Q614. The full-wave rectifier bridges (e.g., includingthe first diode D621, the second diode D622, the third diode D623,and/or the body diodes of the FETs Q612, Q614) may be configured toreceive a voltage developed across the controllably conductive device610 and generate a rectified voltage V_(RECT) at the input of the powersupply.

The power supply 620 may comprise a diode D624 configured to charge abus capacitor C625 from the rectified voltage V_(RECT). The power supply620 may comprise a power converter circuit 626 (e.g., a flybackconverter) configured to receive the bus voltage V_(BUS) and generatethe first and second DC supply voltages V_(CC1), V_(CC2). In addition,the power converter circuit 626 may comprise a linear regulator, a boostconverter, a buck converter, a buck-boost converter, a single-endedprimary-inductance converter (SEPIC), a Ćuk converter, or any othersuitable power converter circuit for generating the first and second DCsupply voltages V_(CC1), V_(CC2).

The control circuit 615 may be configured to monitor one or morevoltages of the power supply 610. The load control device may comprise afirst scaling circuit 627 that may generate a scaled rectified voltagesignal V_(RECT-S) from the rectified voltage V_(RECT) and/or a secondscaling circuit 628 that may generate a scaled bus voltage signalV_(BUS-S) from the bus voltage V_(BUS). For example, the first andsecond scaling circuits 627, 628 may each comprise a resistive dividercircuit. The control circuit 615 may be configured to sample the scaledrectified voltage signal V_(RECT-S) and/or the scaled bus voltage signalV_(BUS-S), for example, using internal analog-to-digital converters(ADCs), in order to determine the magnitude of the rectified voltageV_(RECT) and the bus voltage V_(BUS), respectively. The control circuit615 may be configured to monitor the magnitude of the rectified voltageV_(RECT) and/or the magnitude of the bus voltage V_(BUS) to make surethat the power supply 620 is operating correctly and/or is able tooperate correctly (e.g., as will be described in greater detail below).Since the bus voltage V_(BUS) is generated across the capacitor C625,the magnitude of the bus voltage may change at a slower rate than themagnitude of the rectified voltage V_(RECT) in certain conditions (e.g.,when the magnitude of the rectified voltage may be decreasing rapidlyindicating that the power supply is approaching a condition in which thepower supply 620 may not be able to appropriately generate the first andsecond supply voltages V_(CC1), V_(CC2)). The control circuit 615 may beconfigured to respond to such conditions more quickly using the scaledrectified voltage signal V_(RECT-S) than the scaled bus voltage signalV_(BUS-S). The scaled bus voltage signal V_(BUS-S) may indicatecontinued conditions in which the power supply 620 may not be able toappropriately generate the first and second supply voltages V_(CC1),V_(CC2). The control circuit 615 may be configured to adjust how thecontrollably conductive device 610 is being controlled to try to avoidconditions in which the power supply 6520 may not be able toappropriately generate the first and second supply voltages V_(CC1),V_(CC2) (e.g., as will be described in greater detail below).

The control circuit 615 may be configured to determine times ofzero-crossing points of the AC mains line voltage V_(AC) of the AC powersource 604. The control circuit 615 may then render the FETs Q612, Q614conductive and/or non-conductive at predetermined times (e.g., at afiring time or firing angle) relative to the zero-crossing points of theAC mains line voltage V_(AC) to generate a phase-control voltage V_(PC)using a phase-control dimming technique (e.g., a forward phase-controldimming technique and/or a reverse phase-control dimming technique). Thecontrol circuit 615 may be configured to adjust a phase angle (e.g., aconduction time) of the controllably conductive device 610 eachhalf-cycle to control the amount of power delivered to the lighting load602 and the intensity of the lighting load. For example, the controlcircuit 615 may be configured to adjust a present phase angle θ_(PRES)of the controllably conductive device 610 to adjust the intensity of thelighting load 602 to the desired intensity L_(DES) (e.g., as set by theintensity adjustment actuator of the user interface 617). Using theforward phase-control dimming technique, the control circuit 615 mayrender the controllably conductive device 610 non-conductive at thebeginning of each half cycle, and render the controllably conductivedevice conductive at a firing time (e.g., as determined from the presentphase angle θ_(PRES)) during the half cycle. Using the reversephase-control dimming technique, the control circuit 615 may render thecontrollably conductive device 610 conductive at the beginning of eachhalf cycle, and render the controllably conductive device non-conductiveat a firing time (e.g., as determined from the present phase angleθ_(PRES)) during the half cycle, after which the control circuit maymaintain the controllably conductive device non-conductive for the restof the half cycle.

The load control device 600 may comprise a two-wire zero-cross detectcircuit 630 coupled across the first FET Q612 (e.g., between the hotterminal H and the dimmed hot terminal DH) for generating a two-wirezero-cross signal V_(2WZC). The load control device 600 may alsocomprise a three-wire zero-cross detect circuit 640 coupled between thehot terminal H and the neutral terminal N for generating a three-wirezero-cross signal V_(3WZC). The control circuit 615 may be configured toreceive the two-wire zero-cross signal V_(2WZC) and/or the three-wirezero-cross signal V_(3WZC), and to determine the times of thezero-crossing points of the AC mains line voltage V_(AC) in response tothe two-wire zero-cross signal V_(2WZC) and/or the three-wire zero-crosssignal V_(3WZC). For example, the control circuit 615 may use theforward phase-control dimming technique to control inductive loads, andmay use the reverse phase-control dimming technique to controlcapacitive loads.

The load control device 600 may be programmed by a user duringinstallation to use the forward phase-control dimming technique or thereverse phase-control dimming technique during operation. For example,the user may set the phase-control dimming technique using an advancedprogramming mode. The control circuit 615 may be configured to enter theadvanced programming mode in response to one or more actuations of theactuators of the user interface 617. The control circuit 615 may employa load detection process for determining a load type of lighting load602 and use the phase-control dimming technique that is best suited forthat load type. For example, the control circuit 615 may detect that thelighting load 602 is inductive, and may determine to use the forwardphase-control dimming technique. For example, upon initial power up, thecontrol circuit 615 may begin using the reverse phase-control dimmingtechnique and may monitor the voltage across the lighting load 602 usinga voltage monitor circuit (not shown) during the load detection process.In the event that the control circuit 615 detects an overvoltagecondition (e.g., a voltage spike or ring-up condition) across thelighting load 602, the load control device may determine that thelighting load has inductive characteristics, and may begin using theforward phase-control dimming technique. Otherwise, the control circuit615 may continue to use the reverse-phase control dimming technique.Similarly, upon initial power up, the control circuit 615 may beginusing the forward phase-control dimming technique and may subsequentlydecide to switch to the reverse-phase control dimming technique (e.g.,upon detecting that the lighting load has capacitive characteristics) orto continue to use the forward phase-control dimming technique.

The load control device 600 may comprise a neutral wire detect circuit650 coupled in series with the neutral terminal N (e.g., between thethree-wire zero-cross detect circuit 640 and the neutral terminal N).The neutral wire detect circuit 650 may be configured to generate aneutral wire detect signal V_(NWD) in response to current flowingthrough the three-wire zero-cross detect circuit 640. The controlcircuit 615 may be configured to detect if the neutral terminal N isconnected to the neutral side of the AC power source 604 in response tothe neutral wire detect circuit 650. The control circuit 615 may beconfigured to determine whether to operate in the two-wire mode or thethree-wire mode in response to the neutral wire detect signal V_(NWD).For example, the control circuit 615 may be configured to automaticallydetermine to operate in the two-wire mode in response to detecting thatthe neutral terminal N is not connected to the neutral side of the ACpower source 604 and to operate in the three-wire mode in response todetecting that the neutral terminal N is connected to the neutral sideof the AC power source. For example, the control circuit 615 may beconfigured to automatically determine to operate in the two-wire mode orthe three-wire mode in response to the neutral wire detect signalV_(NWD) during a start-up procedure of the control circuit (e.g., whenpower is first applied to the load control device 600). In addition, thecontrol circuit 615 may monitor the neutral wire detect signal V_(NWD)during normal operation and determine to switch between the two-wiremode and three-wire mode in response to the neutral wire detect signalV_(NWD).

The control circuit 615 may be configured to provide a visual indicationwhen the control circuit decides (e.g., automatically decides) tooperate in the two-wire mode or the three-wire mode in response to theneutral wire detect signal V_(NWD) (e.g., to indicate when the neutralterminal N is connected to the neutral side of the AC power source 604).The control circuit 615 may blink one or more of the visual indicatorsof the user interface 622 when the control circuit decides to operate inthe two-wire mode or the three-wire mode. For example, the controlcircuit 615 may control the user interface 622 to blink twice a topvisual indicator of a vertically-oriented linear array of visualindicators when the control circuit decides to operate in the three-wiremode. The control circuit 615 may be configured to not provide a visualindication when the control circuit decides to operate in the two-wiremode. Since the control circuit 615 automatically decides to operate inthe two-wire mode or the three-wire mode, the visual indication that theload control device 600 is operating in the three-wire mode may beuseful in determining how the load control device is operating.

The control circuit 615 may also be configured to provide a visualindication of the mode (e.g., two-wire mode or three-wire mode) that thecontrol circuit is operating in during the advanced programming mode(e.g., to indicate when the neutral terminal N is connected to theneutral side of the AC power source 604). The control circuit 615 may beconfigured to provide the visual indication of the mode when, forexample, the control circuit is first entering the advanced programmingmode. For example, the control circuit 615 may be configured to blinkone of the visual indicators a first number of times to indicate thetwo-wire mode and second number of times to indicate the three-wiremode. In addition, the control circuit 615 may be configured to providea visual indication of the phase-control dimming technique (e.g., theforward phase-control dimming technique or the reverse phase-controldimming technique) that is presently being used during the advancedprogramming mode. For example, the control circuit 615 may be configuredto blink one of the visual indicators (e.g., a different visualindicator than used to indicate the mode) a first number of times toindicate the forward phase-control dimming technique and second numberof times to indicate the reverse phase-control dimming technique.

The control circuit 615 may be configured to control the FETs Q612, Q614using the forward phase-control dimming technique and/or the reversephase-control dimming technique. When using the forward phase-controldimming technique, the control circuit 615 may render one or both of theFETs Q612, Q614 non-conductive (e.g., to cause the controllablyconductive device 610 to be non-conductive) at the beginning of eachhalf-cycle of the AC mains line voltage, and then render one or both ofthe FETs Q612, Q614 conductive (e.g., to cause the controllablyconductive device 610 to be conductive) at the firing time during thehalf-cycle after which the controllably conductive device 610 may remainconductive until the end of the half-cycle. When using the reversephase-control dimming technique, the control circuit may render one orboth of the FETs Q612, Q614 conductive (e.g., to cause the controllablyconductive device 610 to be conductive) at the beginning of eachhalf-cycle of the AC mains line voltage, and then render one or both ofthe FETs Q612, Q614 non-conductive (e.g., to cause the controllablyconductive device 610 to be non-conductive) at the firing time duringthe half-cycle after which the controllably conductive device 610 mayremain non-conductive until the end of the half-cycle.

The load control device 600 may comprise an impedance circuit 660, suchas a resistive load circuit (e.g., a “dummy” load circuit), fordischarging a capacitance of the lighting load 602, for example, afterthe control circuit 615 renders the FETs Q612, Q614 non-conductive atthe firing time when using the reverse phase-control dimming technique.The impedance circuit 660 may be coupled between the dimmed-hot terminalDH and the neutral terminal N (e.g., in parallel with the lighting load602). The impedance circuit may conduct a discharge current (e.g.,through the dimmed-hot terminal DH, the neutral wire detect circuit 650,and the neutral terminal N) in order to discharge the capacitance of thelighting load 602 after the FETs are rendered non-conductive. Forexample, the impedance circuit 660 may be characterized by a resistanceof approximately 68 kΩ.

The control circuit 615 may configured to determine the firing times forrendering the FETs Q612, Q614 conductive each half-cycle based on thetimes of zero-crossing points of the AC mains line voltage V_(AC) asdetermined from the two-wire zero-cross detect circuit 630 and/or thethree-wire zero-cross detect circuit 640. The two-wire zero-cross detectcircuit 630 may comprise a simple zero-cross detect circuit and maydrive the magnitude of the two-wire zero-cross signal V_(2WZC) lowtowards circuit common when the magnitude of the voltage across thefirst FET Q612 exceeds a predetermined threshold.

The three-wire zero-cross detect circuit 640 may include a filtercircuit 642 (e.g., a half-wave filter circuit) and/or a signalgeneration circuit 644. The filter circuit 642 may comprise a low-passactive filter circuit (e.g., comprising one or more operationalamplifiers), such as a fourth-order Bessel filter. The filter circuit642 and/or the signal generation circuit 644 may be referenced (e.g.,directly referenced) to circuit common of the load control device 600(e.g., circuit common at the junction of the FETs Q612, Q614). Thefilter circuit 642 may be powered by the second supply voltage V_(CC2)(e.g., 12V) generated by the power supply 626. Accordingly, the powersupply 620 may not need to generate another supply voltage (e.g., theisolated DC supply voltage V_(CC3) described in association with FIG. 1)to power the filter circuit 642 and/or the signal generation circuit 644(e.g., the power supply 626 may have a simpler design than the powersupply 120 of the load control device 100 of FIG. 1). The input of thefilter circuit 642 may be coupled to the neutral terminal N via a diodeD646, which may cause the filter circuit to conduct current in thenegative half-cycles of the AC mains line voltage V_(AC) (e.g., conductcurrent only in the negative half-cycles such that the three-wirezero-cross detect circuit 640 operates as a half-wave zero-cross detectcircuit). For example, the filter circuit 642 may conduct current duringthe negative half-cycles of the AC mains line voltage V_(AC) through theneutral terminal N, the diode D646, circuit common, the body diode ofthe FET Q612, and the hot terminal H. During the positive half-cycles ofthe AC mains line voltage V_(AC), the filter circuit 642 may not conductcurrent through the neutral terminal N, the diode D646, circuit common,the body diode of the FET Q612, or the hot terminal H.

The filter circuit 642 may receive a signal that represents the AC mainsline voltage V_(AC), and may generate a filtered signal V_(F). Thefilter circuit 642 may operate to substantially remove from (orattenuate in) the filtered signal V_(F) frequency components of the ACmains line voltage V_(AC) that are above the fundamental frequency. Thefilter circuit 642 may be substantially the same as the circuit shown inFIG. 8A of previously-referenced U.S. Pat. No. 6,091,205. When thefilter circuit receives a half-wave rectified signal through the diodeD646, the filter circuit may not require an input circuit to scale andoffset the AC mains line voltage V_(AC) (e.g., as in the filter circuit142 of the load control device 100 of FIG. 1). The signal generationcircuit 644 (e.g., shown as a signal generator in FIG. 6) may receivethe filtered signal V_(F) and generate the three-wire zero-cross signalV_(3WZC). When the signal generation circuit 644 is coupled to thecircuit common of the load control device 600, the signal generationcircuit may not require an optocoupler circuit at its output forcoupling the three-wire zero-cross signal V_(3WZC) to the controlcircuit 615.

The frequency of the three-wire zero-cross signal V_(3WZC) may beapproximately equal to the frequency of the AC mains line voltageV_(AC). The control circuit 615 may be configured to determine at leastone zero-crossing point during each line cycle of the AC mains linevoltage V_(AC) in response to detecting edges of the three-wirezero-cross signal V_(3WZC). The filter circuit 642 may introduce a phasedelay in the filtered signal V_(F) with respect to the AC mains linevoltage V_(AC). The signal generation circuit 644 may generate edges inthe three-wire zero-cross signal V_(3WZC) (e.g., drive the three-wirezero-cross signal V_(3WZC) low towards circuit common) when themagnitude of the filtered voltage V_(F) exceeds a predeterminedthreshold (e.g., the signal generation circuit 644 may be a simplezero-cross detect circuit). Because of the phase delay between thefiltered signal V_(F) and the AC mains line voltage V_(AC), the edges ofthe three-wire zero-cross signal V_(3WZC) that indicate thezero-crossing points of the AC mains line voltage V_(AC) may be offset(e.g., delayed) from the actual zero-crossing points of the AC mainsline voltage V_(AC). The phase delay may be pre-determined. The controlcircuit 615 may be configured to store a value representing the phasedelay in the memory 628 and process the three-wire zero-cross signalV_(3WZC) by factoring in the phase delay to determine the actual timesof the zero-crossing points of the AC mains line voltage V_(AC).

When operating in the two-wire mode, the power supply 620 may conduct acharging current through the lighting load 602 when the controllablyconductive device 610 is non-conductive each half-cycle. When thecontrol circuit 615 is controlling the intensity of the lighting load602 to the high-end intensity L_(HE), the power supply 620 may have thesmallest amount of time to charge each half-cycle of all points alongthe dimming range of the load control device 600. In some examples(e.g., when controlling the intensity of the lighting load 602 near thehigh-end intensity Urn), the power supply 620 may not be able to conductenough charging current through certain types of lighting loads whilethe controllably conductive device 610 is non-conductive in order toadequately generate the first and second supply voltages V_(CC1),V_(CC2) (e.g., due to the impedances of the lighting loads). In someexamples (e.g., when using the reverse phase-control dimming techniqueto control the FETs Q612, Q614 and/or when operating in the two-wiremode), some types of lighting loads may conduct even less chargingcurrent through the power supply 620 during the times when thecontrollably conductive device 610 is non-conductive.

The control circuit 615 may be configured to execute a plurality ofdifferent power supply protection techniques (e.g., when operating inthe two-wire mode and/or when using the reverse phase-control dimmingtechnique). The control circuit 615 may be configured to monitor themagnitude of the rectified voltage V_(RECT) and/or the magnitude of thebus voltage V_(BUS) to make sure that the power supply 620 is able toproperly generate supply voltages (e.g., the first and second supplyvoltages V_(CC1), V_(CC2)) for powering components of the load controldevice 600. When the magnitude of the rectified voltage V_(RECT) and/orthe magnitude of the bus voltage V_(BUS) drop to a level that isunacceptable to guarantee continued operation of the power supply 620,the control circuit 615 may be configured to adjust how the controlcircuit is controlling the FETs Q612, Q614. For example, the controlcircuit 615 may be configured to adjust (e.g., reduce) the intensity ofthe lighting load 602 (e.g., to increase the amount of time that thepower supply 620 is able to charge while the controllably conductivedevice 610 is non-conductive each half-cycle). As described herein, theintensity of the lighting load 602 may be adjusted (e.g., reduced) byadjusting a present phase angle θ_(PRES) of the controllably conductivedevice 610. Additionally or alternatively, the control circuit 615 maybe configured to adjust (e.g., reduce) the high-end intensity L_(HE) ofthe lighting load 602. Further, the control circuit 615 may determinethat the power supply 620 may be able to charge more effectively throughsome types of lighting loads using the forward phase-control dimmingtechnique. In response to such determination, the control circuit 615may be configured to adjust the type of phase-control dimming techniquebeing used to control the lighting load 602 (e.g., by changing from thereverse phase-control dimming technique to the forward phase-controldimming technique).

FIG. 7 is a state diagram illustrating the operation of a controlcircuit of a load control device (e.g., the control circuit 115 of theload control device 100 of FIG. 1 and/or the control circuit 615 of theload control device 600 of FIG. 6) during an example control procedure700. During the control procedure 700, the control circuit may monitorthe operation of a power supply (e.g., the power supplies 120, 620) toensure that the power supply is able to generate one or more supplyvoltages (e.g., the first and second supply voltages V_(CC1), V_(CC2)).When the power supply is able to appropriately generate the one or moresupply voltages, the control circuit may operate in a normal mode 710.During the normal mode, the control circuit may adjust the intensity ofa lighting load (e.g., the lighting loads 102, 602) to a desiredintensity L_(DES), for example, in response to actuations of one of morebuttons of a user interface (e.g., the user interfaces 117, 617) and/ora message received via a communication circuit (e.g., the communicationcircuits 119, 619). The control circuit may be configured to monitor amagnitude of a bus voltage V_(BUS) across a capacitor of the powersupply (e.g., the bus voltage V_(BUS) across the bus capacitor C625shown in FIG. 6) to determine if the power supply is approaching acondition in which the power supply may not be able to appropriatelygenerate the supply voltages. For example, the control circuit maymonitor the magnitude of the bus voltage V_(BUS) by periodicallysampling the scaled bus voltage signal V_(BUS-S) (e.g., as shown in FIG.6).

The control circuit may be configured to adjust a present phase angleθ_(PRES) of a controllably conductive device (e.g., the controllablyconductive device 110 and/or the controllably conductive device 610) inresponse to the magnitude of the bus voltage V_(BUS). When the magnitudeof the bus voltage V_(BUS) drops to or below a foldback threshold V_(FB)(e.g., approximately 70 volts), the control circuit may operate in afoldback mode 720 in which the control circuit may reduce the presentphase angle θ_(PRES) by a foldback step Δθ_(FB) (e.g., approximately0.7°). For example, the control circuit may be configured toperiodically decrease the present phase angle θ_(PRES) by the foldbackstep Δθ_(FB) (e.g., a foldback amount) at a foldback period T_(FB)(e.g., every 10 milliseconds) while the magnitude of the bus voltageV_(BUS) is less than or equal to the foldback threshold V_(FB) in thefoldback mode 720. The control circuit may cease periodically decreasingthe present phase angle θ_(PRES) by the foldback step Δθ_(FB) at thefoldback period T_(FB) when the magnitude of the bus voltage V_(BUS)rises back above the foldback threshold V. When the magnitude of the busvoltage V_(BUS) rises above a first recovery threshold V_(RV1) (e.g.,approximately 85 volts) while in the foldback state, the control circuitmay operate in a recovery mode 730 in which the control circuit mayincrease the present phase angle θ_(PRES) by a recovery step Δθ_(RV)(e.g., approximately 0.7°). For example, the control circuit may beconfigured to periodically increase the present phase angle θ_(PRES) bythe recovery step Δθ_(RV) (e.g., a recovery amount) at a recovery periodT_(RV) (e.g., every 10 milliseconds) while in the recovery mode 730. Ifthe control circuit increases the present phase angle θ_(PRES) such thatthe intensity of the lighting load is returned to the desired intensityL_(DES), the control circuit may begin operating in the normal state 710again. If the magnitude of the bus voltage V_(BUS) drops to or below thefoldback threshold V_(FB) while in the recovery mode 730, the controlcircuit may return to the foldback mode 720.

While in the foldback mode 720, the control circuit may be configured toturn off the lighting load if the magnitude of the bus voltage V_(BUS)falls even lower (e.g., despite the control circuit periodicallydecreasing the present phase angle θ_(PRES)). For example, when themagnitude of the bus voltage V_(BUS) drops to or below a first shedthreshold V_(SH1) (e.g., approximately 60 volts), the control circuitmay operate in a first shed mode 740 during which the control circuitmay turn off the lighting load. When the magnitude of the bus voltageV_(BUS) rises above a second recovery threshold V_(RV2) (e.g.,approximately 75 volts) while in the first shed mode 740, the controlcircuit may return to the normal mode and may attempt to turn thelighting load back on to the desired intensity L_(DES). The controlcircuit may be configured to store the desired intensity L_(DES) inmemory before turning off the lighting load so that the control circuitmay turn the lighting load back on to the desired intensity L_(DES) whenreturning to the normal mode.

If the magnitude of the bus voltage V_(BUS) drops to or below a secondshed threshold V_(SH2) (e.g., approximately 45 volts) while in the firstshed mode 740, the control circuit may begin to operate in a second shedmode 750 during which the control circuit may turn off one or morestages (e.g., one or more components) of the power supply (e.g., turnoff the power converter circuit 626). If the magnitude of the busvoltage V_(BUS) rises above a third recovery threshold V_(RV3) (e.g.,approximately 45 volts) while in the second shed mode 750, the controlcircuit may return to the first shed mode 740 and turn back on the oneor more stages of the power supply. During the second shed mode 750, themagnitude of the bus voltage V_(BUS) may continue to fall until thecontrol circuit resets. After resetting, the control circuit may returnto the normal state 710 and attempt to turn the lighting load back on tothe desired intensity L_(DES).

As previously mentioned, the power supply may be able to charge moreeffectively through some lighting loads when using the forwardphase-control dimming technique rather than the reverse phase-controldimming technique. When executing the control procedure 700, the controlcircuit may repetitively turn the lighting load off and then back on(e.g., by entering the foldback mode 720, entering the first shed mode740, and then returning to the normal mode 710), which may result in thelighting load blinking or flashing. The control circuit may beconfigured to determine when the lighting load is being repetitivelyturned off and then back on, and to change from using the reversephase-control dimming technique to using the forward phase-controldimming technique.

FIG. 8 is a flowchart of an example phase-control adjustment procedure800 that may be executed by a control circuit of a load control device(e.g., the control circuit 115 of the load control device 100 of FIG. 1and/or the control circuit 615 of the load control device 600 of FIG.6). For example, the control circuit may execute the phase-controladjustment procedure 800 at 810 each time that the control circuitenters the first shed mode 740 from the foldback mode 720 during thecontrol procedure 700 shown in FIG. 7. When executing the phase-controladjustment procedure 800 while using the reverse phase-control dimmingtechnique, the control circuit may use a counter X to keep track of howmany times the lighting load is turned off in response to the magnitudeof the bus voltage V_(BUS) during a monitoring period T_(MON) (e.g.,which may be pre-configured). The control circuit may switch to theforward phase-control dimming technique if the counter X exceeds athreshold X_(TH) (e.g., three). For example, the control circuit maydecide (e.g., only decide) to automatically change from using thereverse phase-control dimming technique to using the forwardphase-control dimming technique when the control circuit is operating inthe two-wire mode (e.g., as determined in response to the neutral wiredetect signal V_(NWD)).

At 812, the control circuit may first turn off the lighting load (e.g.,when entering the first shed mode 740 from the foldback mode 720). Ifthe control circuit is not operating in the two-wire mode at 814 (e.g.,the control circuit is operating in the three-wire mode) or is not usingthe reverse phase-control dimming technique at 816, the phase-controladjustment procedure 800 may simply exit. If the control circuit isoperating in the two-wire mode at 814 and is using the reversephase-control dimming technique at 816, the control circuit mayincrement the counter X at 818 (e.g., by one). If the counter X is lessthan the threshold X_(TH) at 820, the control circuit may initialize acountdown timer to the monitoring period T_(MON) and start countdowntimer counting down at 822, before the phase-control adjustmentprocedure 800 exits. If the countdown timer is already running at 822,the control circuit may simply reset the countdown time to themonitoring period T_(MON). When the counter X is greater than or equalto the threshold X_(TH) at 820, the control circuit may begin to operateusing the forward phase-control technique at 824 and the phase-controladjustment procedure 800 may exit.

FIG. 9 is a flowchart of an example countdown timer procedure 900 thatmay be executed by a control circuit of a load control device (e.g., thecontrol circuit 115 of the load control device 100 of FIG. 1 and/or thecontrol circuit 615 of the load control device 600 of FIG. 6). Forexample, the control circuit may execute the countdown timer procedure900 at 910 in response to the countdown timer that was started at 822 ofthe phase-control adjustment procedure 800. When the countdown timerexpires at 910, the control circuit may reset the counter X to zero at912 and the countdown timer procedure 900 may exit.

FIG. 10 is a flowchart of an example high-end trim adjustment procedure1000 that may be executed by a control circuit of a load control device(e.g., the control circuit 115 of the load control device 100 of FIG. 1and/or the control circuit 615 of the load control device 600 of FIG.6). For example, the control circuit may execute the high-end trimadjustment procedure 1000 periodically (e.g., every 10 milliseconds) at1010. The control circuit may execute the high-end trim adjustmentprocedure 1000 in addition to executing the procedure 700 shown in FIG.7. During the high-end trim adjustment procedure 1000, the controlcircuit may monitor the operation of a power supply (e.g., the powersupplies 120, 620) to ensure that the power supply is able to generateone or more supply voltages (e.g., the first and second supply voltagesV_(CC1), V_(CC2)). The control circuit may be configured to determine(e.g., within a short period of time) if the power supply is approachinga condition in which the power supply may not be able to appropriatelygenerate the supply voltages by monitoring a magnitude of a rectifiedvoltage V_(RECT) that is received by the power supply (e.g., therectified voltage V_(RECT) shown in FIGS. 1 and 6). For example, thecontrol circuit may sample a control signal that indicates the magnitudeof the rectified voltage (e.g., the scaled rectified voltage signalV_(RECT-S) shown in FIG. 6). Since the control circuit is responsive tothe rectified voltage V_(RECT) when executing the high-end trimadjustment procedure 1000, the control circuit may be more responsive toconditions in which the power supply may not be able to appropriatelygenerate the supply voltages than if the control circuit was responsiveto the bus voltage V_(BUS) (e.g., as in the control procedure 700 shownin FIG. 7).

The control circuit may be configured to adjust (e.g., reduce) ahigh-end phase angle θ_(HE) (e.g., a high-end trim) of the load controldevice in response to the magnitude of the bus voltage V_(BUS) duringthe high-end trim adjustment procedure 1000. The high-end phase angleθ_(HE) may define the high-end intensity L_(HE) of the lighting load(e.g., the maximum intensity to which the control circuit may controlthe lighting load). The control circuit may not adjust the present phaseangle θ_(PRES) above the high-end phase angle θ_(HE). Once the high-endphase angle θ_(HE) is reduced by the control circuit during the high-endtrim adjustment procedure 1000, the high-end phase angle θ_(HE) mayremain latched at the reduced level. For example, the high-end phaseangle θ_(HE) may remain latched at the reduced level until power iscycled (e.g., power is disconnected and reconnected) to the load controldevice.

The control circuit may adjust the high-end phase angle θ_(HE) afterreceiving (e.g., only after receiving) a command to control the lightingload (e.g., via the user interfaces 117, 617 and/or the communicationcircuits 119, 619). For example, the control circuit may adjust thehigh-end phase angle θ_(HE) during an adjustment time period T_(ADJ)(e.g., approximately 1500 milliseconds) after receiving a command tocontrol the lighting load (e.g., after an actuation of one or more ofthe actuators of the user interfaces 117, 617). If the control circuitis not within the adjustment time period T_(ADJ) from receiving thecommand to control the lighting load at 1012, the high-end trimadjustment procedure 1000 may simply exit. If the control circuit iswithin the adjustment time period T_(ADJ) from receiving the command tocontrol the lighting load at 1012, the control circuit may determine ifthe present phase angle θ_(PRES) is greater than a minimum phase angleθ_(P-MIN) (e.g., approximately 90°) at 1014. If the present phase angleθ_(PRES) is greater than the minimum phase angle θ_(P-MIN) at 1014, thecontrol circuit may determine if the high-end phase angle θ_(HE) isgreater than a minimum high-end phase angle θ_(HE)-MIN (e.g.,approximately 105°) at 1016. If the present phase angle θ_(PRES) is notgreater than the minimum phase angle θ_(P-MIN) at 1014 or not greaterthan the minimum high-end phase angle θ_(HE-MIN) at 1016, the high-endtrim adjustment procedure 1000 may exit. If the high-end phase angleθ_(HE) is greater than the minimum high-end phase angle θ_(HE-MIN) at1016, the control circuit may continue on to determine if the high-endphase angle θ_(HE) should be adjusted.

The control circuit may set a voltage threshold V_(TH) for determiningif the magnitude of the rectified voltage V_(RECT) is at an unacceptablelevel based on whether the control circuit is operating in the two-wiremode or three-wire mode. For example, if the control circuit isoperating in the two-wire mode at 1018, the control circuit may, at1020, set the voltage threshold V_(TH) equal to a two-wire thresholdV_(TH-2W) (e.g., approximately 83 volts). Detecting that the magnitudeof the scaled rectified voltage signal V_(RECT-S) has dropped below thetwo-wire threshold V_(TH-2W) may indicate a condition in which the powersupply may not be able to appropriately generate the supply voltages. Ifthe control circuit is not operating in the two-wire mode at 1018 (e.g.,is operating in the three-wire mode), the control circuit may, at 1022,set the voltage threshold V_(TH) equal to a three-wire thresholdV_(TH-3W) (e.g., approximately 40 volts). Detecting that the magnitudeof the scaled rectified voltage signal V_(RECT-S) has dropped below thethree-wire threshold V_(TH-3W) may indicate a condition in which one ormore other circuits of the load control device may be able to operateproperly.

At 1024, the control circuit may sample the scaled rectified voltagesignal V_(RECT-S) during a sampling window T_(SMPL) (e.g., 1.5milliseconds). For example, the control circuit may periodically samplethe scaled rectified voltage signal V_(RECT-S) a number M times (e.g.,10 times) over the length of the sampling window T_(SMPL). When usingthe forward phase-control dimming technique, the sampling windowT_(SMPL) may occur before (e.g., immediately before) the firing timewhen a controllably conductive device (e.g., the controllably conductivedevices 110, 610) is rendered conductive. When using the reversephase-control dimming technique, the sampling window T_(SMPL) may occurafter (e.g., immediately after) the firing time when the controllablyconductive device is rendered non-conductive. At 1026, the controlcircuit may determine a number N of samples from the sampling windowT_(SMPL) that exceed the voltage threshold V_(TH) (e.g., as determinedat 1020 and 1022). If the number N of samples that exceed the voltagethreshold V_(TH) is greater than or equal to a threshold NTH (e.g.,seven) at 1028, the high-end trim adjustment procedure 1000 may exit. Ifthe number N of samples that exceed the voltage threshold V_(TH) is notgreater than or equal to the threshold NTH at 1028, the control circuitmay decrease the high-end phase angle θ_(HE) by a high-end reductionstep Δθ_(HE) (e.g., approximately 0.7°) at 1030, before the high-endtrim adjustment procedure 1000 exits. For example, the control circuitmay be configured to periodically decrease the high-end phase angleθ_(HE) by the high-end reduction step Δθ_(HE) every 10 milliseconds(e.g., every time the high-end trim adjustment procedure 1000 isexecuted).

Although features and elements are described herein in particularcombinations, each feature or element can be used alone or in anycombination with the other features and elements. For example, thefunctionality described herein may be described as being performed by aload control device, but may be similarly performed by a hub device or anetwork device. The methods described herein may be implemented in acomputer program, software, or firmware incorporated in acomputer-readable medium for execution by a computer or processor.Examples of computer-readable media include electronic signals(transmitted over wired or wireless connections) and computer-readablestorage media. Examples of computer-readable storage media include, butare not limited to, a read only memory (ROM), a random access memory(RAM), removable disks, and optical media such as CD-ROM disks, anddigital versatile disks (DVDs).

1. A load control device configured to control power delivered from an alternating (AC) power source to an electrical load, the load control device comprising: a hot terminal adapted to be electrically coupled to a hot side of the AC power source; a dimmed-hot terminal adapted to be electrically coupled to the electrical load; a neutral terminal; a first zero-cross detect circuit electrically coupled to between the hot terminal and the dimmed-hot terminal, and configured to detect zero-crossing points of an AC mains line voltage generated by the AC power source; a second zero-cross detect circuit electrically coupled between the hot terminal and the neutral terminal, and configured to detect the zero-crossing points of the AC mains line voltage, the second zero-cross circuit comprising an active filter; a neutral wire detect circuit configured to generate, based on current conducted through the second zero-cross detect circuit, a neutral-wire detect signal indicating whether the neutral terminal is connected to a neutral side of the AC power source; and a control circuit configured to determine whether the load control device should operate in a two-wire mode or a three-wire mode based on the neutral wire detect signal, the two-wire mode corresponding to the neutral terminal not being connected to a neutral side of the AC power source, and the three-wire mode corresponding to the neutral terminal being connected to the neutral side of the AC power source; wherein the control circuit is configured to determine the zero-crossing points of the AC mains line voltage in response to the first zero-cross detect circuit in the two-wire mode and in response to the second zero-cross detect circuit in the three-wire mode.
 2. The load control device of claim 1, further comprising: a controllably conductive device adapted to be coupled in series with the electrical load; wherein the control circuit is configured to render the controllably conductive device conductive and non-conductive to control an amount of power delivered to the electrical load.
 3. The load control device of claim 2, further comprising: a power supply configured to receive a rectified voltage and to generate a supply voltage for powering at least the control circuit by conducting a charging current through the electrical load when the controllably conductive device is non-conductive, the power supply comprising a bus capacitor configured to charge from the rectified voltage through a diode to generate a bus voltage.
 4. The load control device of claim 3, wherein the control circuit configured to adjust the amount of power delivered to the electrical load by adjusting a present phase angle of the controllably conductive device between a low-end phase angle and a high-end phase angle, the control circuit configured to decrease the high-end phase angle when a magnitude of the rectified voltage is less than a first threshold, and decrease the present phase angle when a magnitude of the bus voltage is less than a second threshold.
 5. The load control device of claim 4, wherein the control circuit is configured to turn off the electrical load when the magnitude of the bus voltage falls below a third threshold, the third threshold being less than the second threshold.
 6. The load control device of claim 5, wherein the control circuit is configured to: turn on the electrical load when the magnitude of the bus voltage rises above a fourth threshold that is greater than the third threshold; repetitively turn off and on the electrical load in response to the magnitude of the bus voltage; count the number of times that the electrical load is turned off in response to the magnitude of the bus voltage; and change from a reverse phase-control dimming technique to a forward phase-control dimming technique when the number of times that the electrical load is turned off exceeds a fifth threshold.
 7. The load control device of claim 2, wherein the control circuit is configured to control the controllably conductive device to control the load current conducted through the electrical load using a reverse phase-control technique.
 8. The load control device of claim 7, wherein the control circuit is further configured to determine a phase delay between an output of the active filter and the AC mains line voltage, the control circuit further configured to adjust the zero-crossing points detected by the second zero-cross signal based on the phase delay.
 9. The load control device of claim 8, wherein the phase delay is predetermined and stored in a memory of the load control device.
 10. The load control device of claim 2, wherein the active filter comprises a half-wave filter circuit configured to conduct current through the controllably conductive device during negative half-cycles of the AC mains line voltage and not conduct current through the controllably conductive device during positive half-cycles of the AC mains line voltage.
 11. The load control device of claim 10, wherein the controllably conductive device comprises two semiconductor switches coupled together at a junction, wherein the active filter is referenced to a circuit common at the junction of the semiconductor switches.
 12. The load control device of claim 11, wherein the active filter is configured to conduct current through one of the semiconductor switches during the negative half-cycles of the AC mains line voltage and through neither of semiconductor switches during the positive half-cycles of the AC mains line voltage.
 13. The load control device of claim 2, wherein the active filter comprises a full-wave filter circuit coupled between the hot terminal and the neutral terminal and configured to conduct current through the controllably conductive device during positive and negative half-cycles of the AC mains line voltage.
 14. The load control device of claim 13, further comprising: a power supply configured to generate an isolated supply voltage for powering the full-wave filter circuit.
 15. The load control device of claim 2, further comprising: an impedance circuit coupled in parallel with the electrical load; wherein the control circuit is configured to render the controllably conductive device conductive and non-conductive to control a load current conducted through the electrical load, the impedance circuit configured to discharge a capacitance of the electrical load when the controllably conductive device is rendered non-conductive.
 16. The load control device of claim 1, wherein the control circuit is configured to determine whether the load control device should operate in the two-wire mode or the three-wire mode based on the neutral wire detect signal during a start-up routine of the load control device.
 17. The load control device of claim 16, wherein, during the start-up routine, the control circuit is configured to: determine that the load control device is operating in the two-wire mode; detect a number of transitions of a magnitude of the neutral wire detect signal from a first magnitude to a second magnitude within a predetermined number of line cycles of the AC mains line voltage, the number of transitions being equal to or above a threshold; and switch the load control device to the three-wire mode.
 18. The load control device of claim 16, wherein, during the start-up routine, the control circuit is configured to: determine that the load control device is operating in the three-wire mode; detect a number of transitions of a magnitude of the neutral wire detect signal from a first magnitude to a second magnitude within a predetermined number of line cycles of the AC mains line voltage, the number of transitions being below a threshold; and switch the load control device to the two-wire mode.
 19. The load control device of claim 1, wherein the control circuit is configured to determine whether the load control device should operate in the two-wire mode or the three-wire mode based on changes in a magnitude of the neutral wire detect signal within a predetermined number of line cycles of the AC mains line voltage.
 20. The load control device of claim 19, wherein, in order to determine whether the load control device should operate in the two-wire mode or the three-wire mode, the control circuit is configured to: count a number of times within a predetermined number of line cycles of the AC mains line voltage that the magnitude of the neutral wire detect signal transitions from a first magnitude to a second magnitude; determine whether the number of transitions is above a threshold; based on a determination that the number of transitions is equal to or above the threshold, control the load control device to operate in the three-wire mode; and based on a determination that the number of transitions is below the threshold, control the load control device to operate in the two-wire mode.
 21. The load control device of claim 1, wherein, during the two-wire mode, the neutral wire detect circuit is configured to maintain a magnitude of the neutral wire detect signal, and, during the three-wire mode, the neutral wire detect circuit is configured to change the magnitude of the neutral wire detect signal between a first magnitude and a second magnitude over one or more line cycles of the AC mains line voltage.
 22. The load control device of claim 1, wherein the neutral wire detect circuit is coupled electrically between the second zero-cross detect circuit and the neutral terminal.
 23. The load control device of claim 1, wherein the active filter is configured to remove one or more frequency components of the AC mains line voltage that are above a frequency threshold.
 24. The load control device of claim 1, further comprising: a power supply capable of conducting a charging current through the electrical load; and a switching circuit configured to be rendered conductive and non-conductive to control when the charging current is conducted through the electrical load; wherein the control circuit is configured to render the switching circuit conductive during the two-wire mode and non-conductive during the three-wire mode so that, during the two-wire mode, the charging current is conducted through the electrical load during positive half-cycles of the AC mains line voltage, and, during the three-wire mode, the charging current is not conducted through the electrical load during the positive half-cycles of the AC mains line voltage.
 25. The load control device of claim 1, further comprising: a user interface comprising one or visual indicators; wherein the control circuit is configured to control the user interface to provide a visual indication of when the load control device is operating in the three-wire mode. 26-100. (canceled) 